Pulse width modulator

ABSTRACT

A switch-modulator for a radio-frequency power amplifier, arranged to modulate the I-signal and the Q-signal of the complex components (I+j·Q) separately in an I-signal part and a Q-signal part in order to create a modulated I-signal pulse sequence and a modulated Q-signal pulse sequence, wherein the modulation comprises a time-shift of the pulse positions within a sample interval.

TECHNICAL FIELD

The present invention relates to a switch-modulator for a switched-moderadio-frequency power amplifier and to an integrated circuit comprisingsaid switch-modulator, as well as to a method of switch-modulating aradio frequency power amplifier.

BACKGROUND

Modern radio communications system (e.g. WCDMA, Wideband Code DivisionMultiple Access) utilize digital modulation schemes in which both theamplitude and the phase of a complex base band signal are modulated ontothe envelope of a Radio Frequency (RF) carrier, thus constituting a bandpass RF signal. One way of a achieving such a band pass RF signal is touse Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM) ofa switched-mode power amplifier operating at the carrier frequency.

In a switched-mode power amplifier, the power transistors are either ina fully conducting ON-state or in a completely non-conducting OFF-state,and a switched-mode power amplifier may achieve a much higher powerefficiency (ideally up to 100%) than a linear power amplifier.

In conventional PWM (Pulse Width Modulation), the amplitude of a signalis mapped onto the width of a pulse at each sample, and a single pulseis transmitted by the modulator for each incoming sample. Further, inconventional PPM (Pulse Position Modulation), the phase information ofthe signal is mapped onto the position of the pulse, and PPM (PulsePosition Modulation) may be used together with PWM (Pulse WidthModulation) in a PWM/PPM in order to create a pulse sequencerepresenting both the amplitude and the phase of the signal. FIG. 2illustrates how the amplitude of a signal 1 sampled at Ts0 is mappedonto the width, i.e. duration, of a modulated pulse 3, and the phase ofthe signal sample is mapped onto the position of the pulse 3 within thetime interval between two samples Ts=Ts1−Ts0, i.e. the sampling intervalor sampling period.

Although the use of switched-mode RF amplifiers is still limited due tothe high switching frequencies needed, the generation of the necessaryband pass RF signal using PWM/PPM only requires switching at the carrierfrequency and not a multiple of this frequency, as in the case of e.g.band pass Delta-Sigma modulation or low pass PWM. Thereby, the necessarydigital and analogue circuitry for a PWM/PPM may be implemented as anintegrated circuit, e.g. in the form of an RF ASIC (Radio FrequencyApplication Specific Integrated Circuit).

FIG. 1 is a block diagram illustrating a conventional conceptualswitched-mode architecture, consisting of any suitable switch-modulator2 arranged to modulate a base band input signal to present apulse-sequence 3 forming a binary-level signal with correct switchingpositions to a power amplifier 4. Thereafter, the amplified pulsesequence 5 is filtered by a properly designed filter 6 tuned around thecarrier frequency in order to filter out a correct amplified radiofrequency output signal 7.

The technique explained in FIG. 2 is applied in the conventionalarrangement illustrated in FIG. 3, in which the Cartesian coordinates ofa base band signal I+jQ, i.e. the I-signal and the Q-signal, areconverted into Polar coordinates by the converter 10. Theamplitude-signal, A, and the phase-signal, φ, representing the Polarcoordinates are modulated by a conventional, combined PWM/PPM 8, bywhich the amplitude-signal is mapped on the width of a pulse and thephase-signal is mapped on the position of said pulse within the samplingperiod of the baseband signal. Since the mapping of the input amplitudeonto a pulse width is a non-linear function, i.e. a sine-function, aninverse (i.e. arcsine) pre-distorter is needed to obtain a linearoutput, and this correction is pre-calculated in the correctingcalculator 11 in the illustrated arrangement. FIG. 4 further indicatesthe pulse sequence 3 created by the combined PWM/PPM 8 representing boththe amplitude and phase of the base band signal, as described above.Thereafter, the pulse sequence 3 is amplified by the power amplifier 4,and the amplified pulse sequence 5 is filtered by the band pass filter6, resulting in an amplified base band signal 7 on the output.

Related art within the technical field is disclosed e.g. inUS2004/0246060, which describes a modulator for generating a two-levelsignal suitable for amplification by a switching mode power amplifier,such as a Class D amplifier.

However, the above-described conventional arrangements and related artinvolve several drawbacks. For example, a combined pulse width—and pulseposition-modulation with a fixed sample period, Ts, may lead to“wrap-around”, or phase jump, of a pulse when the phase mapping extendsover the +/−180 degree border. This “wrap-around” is illustrated in FIG.4, in which the pulse representing the sample at Ts0 is “wrapped” withinthe sample period, since this first pulse cannot extend over to the nextsample interval. Instead, a second pulse will be transmitted during thenext interval, and this second pulse will represent the amplitude andphase of the second sample, at Ts1. This phase jump may also lead tomissing or wrong pulse widths at the phase jump position.

It is further known within this technical field to combine theabove-described PWM (pulse-width modulation) and PPM (pulse-positionmodulation) with band pass Delta-Sigma (DS) modulation. However, bandpass DSM involves some drawbacks, such as a high out-of-band noise and avery high switching frequency. Normally, the sampling frequencyfs=4·carrier frequency, such as in an fs/4 band pass DS-modulator.

A drawback with the combined PW/PP-modulation is the time granularity ofa digitally defined pulse width and position, which restricts theachievable dynamic range due to quantization noise. At least 512 levelswould be required for the width or positioning in order to reach 60-70dB dynamic range, and this requires a clock frequency and a speed of thedigital circuitry that is not achievable today.

Thus, it still presents a problem to achieve a switch-modulatorarchitecture for a switched-mode radio-frequency power amplifierenabling linear amplification of radio-frequency signals over a largebandwidth with a high dynamic range and without “wrap-around” problems,suitable for implementation as an integrated circuit.

SUMMARY

An object of the present invention is to address the problem outlinedabove, and to provide an improved switch-modulator architecture for aswitched-mode radio frequency power amplifier. This object and othersare achieved by the switch-modulator, the integrated circuit and themethod of switch-modulating, according to the appended claims.

According to a first aspect, the invention relates to a switch-modulatorfor a switched-mode radio-frequency power amplifier, saidswitch-modulator arranged to map the phase and amplitude of an inputcomplex baseband signal represented by the components I and Q onto amodulated output pulse sequence. The switch-modulator is arranged tosample and modulate the I-signal and the Q-signal separately, themodulation comprising a time-shift of the pulse positions within asampling interval. The switch-modulator comprises a first input to aseparate I-signal path in the switch-modulator, a second input to aseparate Q-signal path in the switch-modulator, and a third input to aquadrature clock generator in the switch-modulator. The quadrature clockgenerator is arranged to generate an I-clock signal and a Q-clock signalwith a quadrature-shifted phase-relationship in order to delay amodulated I-signal pulse sequence relative a modulated Q-signal pulsesequence and to generate the time-shift of the pulse positions in saidpulse sequences.

Said generated I-clock signal and Q-clock signal may be sinusoidal,generating an arcsine pre-distortion.

This switch modulator-configuration facilitates implementation as anintegrated circuit, e.g. on an RF-ASIC (Radio Frequency-ApplicationSpecific Integrated Circuit), applicable in various differentswitched-mode radio frequency power amplifier arrangements. By usingseparate I-signal-and Q-signal paths through the switch-modulator andencoding the total amplitude and phase information of a radio-frequencysignal in a properly time-shifted pulse sequence, wrap-around andphase-jumps are avoided, and the radio architecture of the switched-moderadio frequency power amplifier is simplified, achieving a large powerefficiency and dynamic range.

The modulation of the I-signal and the Q-signal may comprise mapping ofthe sample amplitude of the I-signal and the Q-signal on thedifferential time position of two balanced 50%-duty cycle I-signal pulsesequences and two balanced 50%-duty cycle Q-signal pulse sequencesrespectively.

Alternatively, the modulation comprises mapping of the sample amplitudeon the pulse width, and time-shifting the pulses corresponding tonegative sample values relative the pulses corresponding to positivesample values onto a modulated unbalanced I-signal pulse sequence and amodulated unbalanced Q-signal pulse sequence, respectively, and theI-clock signal and said Q-clock signal may be arranged to generatecorrect time-shifted positions and pulse width for the pulses in saidpulse sequences. The switch-modulator may further comprise a combinerarranged to generate a combined modulated pulse sequence, representingthe complex I+j·Q-signal, from the modulated unbalanced I-signal pulsesequence and the modulated unbalanced Q-signal pulse sequence.

The time-shift of the negative sample pulses relative the positivesample pulses may correspond to 0.5 of a sampling interval Ts, and thedelay of the I-signal output pulse sequence relative the Q-signal outputpulse sequence may be 0.25 of a sampling interval Ts, which correspondsto a 90-degree phase-shift and simplifies the combination into a complexI+j·Q-signal.

The switch-modulator may further comprise two I-signal comparators forgenerating two differentially time-shifted balanced I-signal outputpulse sequences from the I-clock signal and from two opposite phase andchopped I-signals, and two Q-signal comparators for generating twodifferentially time-shifted balanced Q-signal output pulse sequencesfrom the Q-clock signal and from two opposite phase chopped Q-signals.It may also comprise two I-signal outputs for the differentiallytime-shifted balanced modulated I-signal pulse sequences and twoQ-signal outputs for the differentially time-shifted balanced modulatedQ-signal pulse sequences. Additionally, the switch-modulator maycomprise an I-signal-gate for generating an unbalanced I-signal outputpulse sequence from said two differentially time-shifted balancedI-signal output pulse sequences, and an Q-signal-gate for generating anunbalanced Q-signal output pulse sequence from said two differentiallytime-shifted balanced Q-signal output pulse sequences, as well as anI-signal output for the unbalanced modulated I-signal pulse sequence andan Q-signal output for an unbalanced modulated Q-signal pulse sequence.

The switch-modulator may further comprise a Cartesian-to-polar-converterfor converting the I-signal and the Q-signal representing an inputbaseband signal into an A(t)-signal representing the amplitude of saidinput baseband signal and connected to said first I-signal input, andinto a phase modulated ω(t)-signal representing the phase of the inputbaseband signal and connected to said third clock input.

Alternatively, the switch-modulator may be provided with a combiner foradding the unbalanced I-signal output pulse sequence to the unbalancedQ-signal output pulse sequence, resulting in a combined modulated pulsesequence representing a modulated input baseband signal I+j·Q, of whichsaid combiner may comprise an OR-gate, and the switch modulator mayfurther comprise a signal-output for the modulated complex I+jQ-pulsesequence.

According to a second aspect, the invention relates to an integratedcircuit provided with a switch-modulator according to theabove-described first aspect.

According to a third aspect, the invention relates to a method ofswitch-modulating a radio-frequency power amplifier, in which method theinput signal is represented by the I-signal and Q-signal of the complexcomponents (I+j·Q). The method comprises the steps of:

-   -   Sampling and pulse width modulating the I-signal and the        Q-signal separately to create a modulated I-signal pulse        sequence and a modulated Q-signal pulse sequence;    -   Time shifting the pulses corresponding to negative sample values        relative the pulses corresponding to positive sample values;    -   Delaying each pulse of the I-signal pulse sequence by        introducing a delaying time shift;    -   Combining the I-signal pulse sequence and the Q-signal pulse        sequence before amplification of the combined pulse sequence.

The method may comprise the additional step of restricting thepeak-values of the input I-signal and Q-signal to avoid overlappingpulses.

The pulse width modulation may comprise mapping of the sample amplitudeon the width of a pulse of a modulated pulse sequence, and the delayingtime shift in the I-branch may be 0.25 Ts, which corresponds to a90-degree phase shift, and the time shift of the negative sample pulsesrelative the positive sample pulses may be 0.5 Ts.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail and withreference to the accompanying drawings, in which:

FIG. 1 is a block diagram schematically illustrating a conventionalswitch-modulated amplifier arrangement;

FIG. 2 illustrates mapping of the amplitude and phase of a signal onto apulse;

FIG. 3 is a block diagram illustrating a conventional arrangement of acombined pulse-width modulator and pulse position modulator;

FIG. 4 illustrates “wrap around” that may occur when the phase of asignal is mapped onto the position of a pulse within a fixed samplingperiod;

FIG. 5 is a block diagram illustrating a first example of aswitch-modulated amplifier arrangement;

FIG. 6 is a block diagram illustrating a second example of aswitch-modulated amplifier arrangement

FIG. 7 is a block diagram illustrating a third example of aswitch-modulated amplifier arrangement;

FIG. 8 illustrates mapping of positive or negative samples of theI-branch- and the Q-branch signals, according to an exemplary embodimentof this invention;

FIG. 9 illustrates Cartesian pulse width modulation for unipolar andbipolar pulses;

FIG. 10 shows a first exemplary switched-mode RF-amplifier arrangementcomprising a switch-modulator according to this invention;

FIG. 11 shows a second exemplary switched-mode RF-amplifier arrangementcomprising a switch-modulator according to this invention;

FIG. 12 shows a third exemplary switched-mode RF-amplifier arrangementcomprising a switch-modulator according to this invention;

FIG. 13 shows a fourth exemplary switched-mode RF-amplifier arrangementcomprising a switch-modulator according to this invention;

FIG. 14 shows a fifth exemplary switched-mode RF-amplifier arrangementcomprising a switch-modulator according to this invention;

FIG. 15 is a block diagram of a switch-modulator according to a firstembodiment of this invention;

FIG. 16 illustrates pulse sequences according to FIG. 15;

FIG. 17 is a block diagram of a switch-modulator according to a secondand third embodiment of this invention;

FIG. 18 illustrates pulse sequences according to FIG. 17;

FIG. 19 illustrates a band-pass filtered amplified radio frequencysignal achieved by a switched-mode amplifier arrangement comprising aswitch-modulator according to this invention, and

FIG. 20 is a flow chart of the method of switch-modulating a RF-poweramplifier, according to the third example of a switch-modulatedamplifier arrangement.

DETAILED DESCRIPTION

In the following description, specific details are set forth, such as aparticular architecture and sequences of steps in order to provide athorough understanding of the present invention. However, it is apparentto a person skilled in the art that the present invention may bepractised in other embodiments that may depart from these specificdetails.

Moreover, it is apparent that the described functions may be implementedusing software functioning in conjunction with a programmedmicroprocessor or a general purpose computer, and/or using anapplication-specific integrated circuit. Where the invention isdescribed in the form of a method, the invention may also be embodied ina computer program product, as well as in a system comprising a computerprocessor and a memory, wherein the memory is encoded with one or moreprograms that may perform the described functions.

This invention relates to a switch-modulator for a switched-moderadio-frequency power amplifier, and the switch-modulator is capable ofmapping information regarding both the amplitude and the phase of thebase band signal onto a modulated output pulse sequence. Instead ofmapping the phase on the position of the pulse between two samples, as aconventional PWM/PPM (pulse width modulator/pulse position modulator), aswitch-modulator according to this invention comprises separate inputsand paths for the I-signal and the Q-signal, respectively, of an inputbaseband signal. The I-signal and the Q-signal represents the Cartesiancoordinates of a complex number, expressed as I+j·Q, of which the realcomponent I represents the In-phase component, i.e. the I-signal, andthe imaginary component Q represents the Quadrature-phase component,i.e. the Q-signal, of the signal. The switch-modulator according to thisinvention comprises an I-signal part and a Q-signal part arranged tosample and modulate the I-signal and the Q-signal separately, and themodulation comprises a time-shift of the pulse positions within asampling interval. Further, in order to forward the complex number I+j·Qrepresenting the base band signal, and not the real part (I) and theimaginary part (Q) separately, the two signals will be transformed intoone signal corresponding to I+j·Q, either before or after theamplification of the separate signals. Since a multiplication by “j”corresponds to a phase-shift of 90° and a time-shift of 0.25·timeperiod, the switch-modulator according to his invention performs adelay, i.e. a time-shift, of the I-branch by 0.25 Ts, of which thesampling period Ts is the sampling period at radio frequency (RF).Thereafter, the signals are combined in order to achieve I+j·Q, andfinally the signal will be filtered in order to create a correct,amplified radio-frequency signal.

The above-described switch-modulator according to this invention enablesa circuit design that is suitable for implementing as an integratedcircuit. By using separate I- and Q-paths through the modulator andmaintaining a proper time-shift of the binary signals in order to encodethe total phase information in an amplified radio-frequency signal, thewrap-around and phase-jump problem is solved and the total radioarchitecture is simplified, thereby facilitating implementation on anRF-ASIC (Radio Frequency-Application Specific Integrated Circuit).

FIG. 5 shows a first exemplary switched-mode amplifier arrangement,comprising a switch modulator 15 according to a first embodiment of thisinvention, capable of modulating the I-signal branch 1 and of theQ-signal branch 9 of a base band signal separately by mapping theamplitude of each sample on differentially time-shifted positions of two50% duty cycle pulses. The modulator comprises an I-signal part 2 a, aQ-signal part 2 b, and a delay unit 12, arranged to delay the I-signalwith 0.25 Ts.

The resulting balanced, i.e. in a time-shift balanced sense, pulsesequences 3 a, 3 b, 3 c, 3 d from the switch modulator 15 are amplifiedby four functionally separate power amplifiers 4 a, 4 b, 4 c,4 d, andthe amplified signals are first pair-wise differentially combined bysuitable combiners, thereby creating pulse-width modulated bipolar I-and Q pulse sequences. The bipolar I and Q pulse sequences are combinedby a suitable combiner 13, creating a combined amplified pulse sequence5, which is filtered by a suitable band-pass filter 6, creating acorrect amplified band pass signal 7.

FIG. 6 shows a second exemplary switched-mode amplifier arrangement,comprising a switch-modulator 15 according to a second embodiment ofthis invention, performing pulse-width modulation of the I-signal branch1 and of the Q-signal branch 9 of a base band signal separately bymapping the amplitude of each sample on the width of a pulse, and bytime-shifting the sample position and pulses of negative sample valuesrelative the sample position and pulses of positive sample values with0.5 Ts. The modulator comprises an I-signal part 2 a, a Q-signal part 2b, and a delay unit 12 arranged to delay the I-signal with 0.25 Ts.

The resulting two separate unbalanced, i.e. in the sense of a signal notcomprising a balanced time-shift, pulse sequences 3 e, 3 f from theswitch-modulator 15 are amplified by two functionally separate poweramplifiers 4 a, 4 b, and the amplified signals are combined by asuitable combiner 13, thereby creating a combined amplified pulsesequence 5, which is filtered by a suitable band-pass filter 6, creatinga correct amplified band-pass signal 7.

FIG. 7 shows a third exemplary switched-mode amplifier arrangement,comprising a switch-modulator 15 according to a third embodiment of thisinvention, performing pulse-width modulation of the I-signal branch 1and of the Q-signal branch 9 of a base band signal separately by mappingthe amplitude of each sample on the width of a pulse, and bytime-shifting the sample position and pulses of negative sample valuesrelative the sample position and pulses of positive sample values with0.5 Ts. The switch-modulator comprises an I-signal part 2 a, a Q-signalpart 2 b, a delay unit 12 arranged to delay the I-signal, and a combiner13 arranged to combine the modulated and delayed I-signal with themodulated Q-signal within the switch-modulator, in order to deliver acombined modulated I+jQ pulse sequence 3 g at the output. Thereafter,the combined pulse sequence 3 g from the switch-modulator can beamplified by a single power amplifier 4, thereby creating a combinedamplified pulse sequence 5, which is filtered by a suitable band-passfilter 6, creating a correct amplified band-pass signal 7.

Thus, according to the above-described first arrangement in FIG. 5, fourfunctionally separate power amplifiers 4 a, 4 b, 4 c, 4 d are used toamplify the four separate pulse sequences 3 a, 3 b, 3 c, 3 d, of which 3a, 3 b corresponds to the modulated and delayed I-signal, and 3 c, 3 dcorresponds to the modulated Q-signal. The amplified signals arepair-wise differentially combined by suitable combiners, therebycreating pulse-width modulated bipolar I and Q pulse sequences. For thisbipolar combined pulse sequence, having three levels, the positions andthe polarity of the pulses encode the signs of the two input signals Iand Q, according to the following exemplary Table 1:

Signal value Positions within T Polarity pattern Positive Q 0 T 0.5 T +− Negative Q 0 T 0.5 T − + Positive I 0.25 T 0.75 T + − Negative I 0.25T 0.75 T − +

Thus, in this first embodiment of the switch-modulator 15 according tothe invention the amplitude and phase information is coded by thedifferential time positions of two 50% duty cycle pulses relative thefixed position defined for each of two separate I- and Q-signals, andsubsequently by the pulse width and polarity pattern of the pulsesresulting from the differential combination of two time-shifted 50% dutycycle pulses into one bipolar pulse sequence after power amplification.

According to the second arrangement in FIG. 6, two functionally separatepower amplifiers 4 a, 4 b are used to amplify the two separate pulsesequences 3 e, 3 f, of which 3 e corresponds to the modulated anddelayed in-phase signal, I-signal, and 3 f corresponds to the modulatedquadrature-phase signal, Q-signal.

According to the third arrangement, illustrated in FIG. 7, only onepower amplifier 4 is needed in order to amplify the combined pulsesequence 3 from the switch-modulator 15. In order to obtain a correctamplified output radio frequency signal, the combining of the In-phase(I) and the Quadrature-phase (Q) signals is preceded by a delay of theI-signal by approximately a quarter of a sample interval (i.e. by 0.25Ts) after the pulse width modulation in the functional delay unit 12.Thereby, the pulses of the I-signal pulse sequence are time-shifted by0.25 Ts within the sampling interval, such that the positive sample maybe placed at 0.5 Ts and the negative sample pulses at 1 Ts, while theun-delayed Q-signal sample pulses may be placed at 0.25 Ts and 0.75 Ts,respectively. A delay by a quarter of a sample interval corresponds to a90-degree phase shift, which simplifies the combination of the twoorthogonal components into the complex representation I+j·Q.

FIG. 8 illustrates the modulation according to an exemplary embodimentof the switch-modulator according to the second and third embodiment ofthis invention, involving separate pulse width modulation of theI-signal and the Q-signal, forming a unipolar modulated pulse sequence.The I-branch is delayed by 0.25 Ts relative the Q-branch, resulting inthat a sample in the I-branch at Ts0 is placed on 0.5 Ts, while a samplein the Q-branch at Ts0 is placed on 0.25 Ts. Further, a pulse associatedwith the positive amplitude at Ts0 is placed in the first half of thesampling period, i.e. at 0.5 Ts and 0.25 Ts, in the I-branch and in theQ-branch, respectively, while a pulse associated with the negativeamplitude at Ts1 is placed in the second half, i.e. at 1 Ts and at 0.75Ts in the I-branch and Q-branch, respectively. By means of theillustrated concept, a complex signal is transformed into a real signalby assigning the pulses to slightly different positions within thesample interval Ts. Obviously, the described positions are only examplesof suitable positions within the sample interval, and severalalternative positions are possible, provided that the I-signal samplepulse is delayed with 0.25 Ts relative the Q-signal sample pulse, andthat the negative sample pulses are displaced with 0.5 Ts relative thepositive sample pulses. For a unipolar pulse sequence, i.e. havingbinary levels, the position of the pulses depends on the sign of the twoinput signals I and Q, according to the following exemplary Table 2:

Signal value Position within T Positive Q 0 T Negative Q 0.5 T PositiveI 0.25 T Negative I 0.75 T

Thus, according to the second and third embodiments of theswitch-modulator according to this invention, the amplitude and phaseinformation is coded by the width of each pulse in two separatetime-shifted I- and Q-signal pulse sequences, and by the positioning ofthe pulses on fixed time-shifted positions within the sample period, T.

FIG. 9 illustrates Cartesian pulse width modulation of positive ornegative samples of the I-branch- and the Q-branch signals onto aresulting unipolar and a bipolar pulse sequence, respectively.

A switch-modulator 15 according to this invention comprises an I-signalpart 2 a and a Q-signal part 2 b, the I-signal part having a first inputto the I-branch and the Q-signal part having a second input to theQ-branch. The switch-modulator 15 is also provided with a third inputfor a clock signal at the intended carrier frequency, or at the centrefrequency of the pass-band of the radio frequency signal, if this iscomposed of several modulated carriers. In accordance with the firstembodiment, the output of each of the I- and Q-branch consists ofbalanced outputs in the form of two separate 50% duty cycle binarysquare pulse sequences, properly time-shifted in relation to each otherin order to encode the amplitude of I and Q branches respectively, alsoincluding the needed arcsine inverse pre-distortion. The I- and Q-branchoutputs are also properly time-shifted in relation to each other inorder to represent the quadrature position of the Q-signal (+90degrees), enabling a simplified combination of the I- and Q-branchoutputs, either within the switch-modulator and before theamplification, or after the amplification.

The outputs of the I- and Q-branch may, alternatively, in accordancewith the second a third embodiments, consist of outputs in the form of asingle (unbalanced) modulated binary square wave pulse sequence,requiring a less complicated radio architecture.

FIG. 10 illustrates an arrangement of a switched-mode radio-frequencypower amplifier, comprising a switch-modulator 15 according to a firstembodiment of this invention, having differentially time-shiftedbalanced outputs from the switch-modulator 15. The I-signal part of theswitch-modulator 15 comprises a first input for the I-signal of abaseband signal, an I-signal path through the switch-modulator and twobalanced I-signal outputs of a modulated unipolar square pulse sequence.The two balanced output pulse sequences have properly differentiallytime-shifted 50% duty cycle-pulses within each sampling period in orderto encode the amplitude of the I-signal, including the required arcsinepre-distortion, and the two balanced output pulse sequences areamplified in power amplifiers 4 a, 4 b and combined in a differentialcombiner, creating a bipolar I-signal pulse sequence. Similarly, theQ-signal part of the switch-modulator 15 comprises a second input forthe Q-signal of a baseband signal, a Q-signal path through theswitch-modulator 15 and two balanced Q-signal outputs of a properlydifferentially time-shifted unipolar pulse sequence. Additionally, theI-signal output pulse sequences and the Q-signal output pulse sequencesare properly time-shifted in relation to each other in order torepresent the quadrature position of the Q-signal. The two balancedQ-outputs are amplified in power amplifiers 4 c, 4 d and combined in adifferential combiner, creating a bipolar Q-signal pulse sequence.Thereafter, the amplified bipolar I-pulse sequence and the bipolarQ-pulse sequence are combined in combiner 13, creating the amplifiedI+jQ-signal, which is filtered in a suitable band-pass filter 6. Theswitch-modulator 15 further comprises a third input for the clock signalat the carrier frequency.

FIG. 11 illustrates an arrangement of a switched-mode radio-frequencypower amplifier, comprising a switch-modulator 15 according to a secondembodiment of this invention, having unbalanced outputs. The I-signalpart of the switch-modulator 15 comprises a first input for the I-signalof a baseband signal, an I-signal path through the switch-modulator andone unbalanced I-signal output of a modulated unipolar square pulsesequence 3 e. The I-signal output pulse sequences has properly pulsewidth modulated and time-shifted pulses within each sampling period, inorder to encode the amplitude of the I-signal, including the requiredarcsine pre-distortion, and is amplified in the power amplifier 4 a.Similarly, the Q-signal part of the switch-modulator 15 comprises asecond input for the Q-signal of a baseband signal, a Q-signal paththrough the switch-modulator 15 and an unbalanced Q-signal output of apulse width modulated and properly time-shifted unipolar pulse sequence3 f, which is amplified in the power amplifier 4 b. Additionally, theI-signal output pulse sequence 3 e and the Q-signal output pulsesequence 3 f are properly time-shifted in relation to each other inorder to represent the quadrature position of the Q-signal. Thereafter,the amplified I-pulse sequence and the Q-pulse sequence are combined ina suitable combiner 13, creating a combined unipolar amplifiedI+j-Q-signal, which is filtered in a suitable band-pass filter 6. Theswitch-modulator 15 further comprises a third input for the clock signalat the carrier frequency.

FIGS. 12 and 13 illustrates a switched-mode amplifier arrangementcomprising a switch-modulator 15 according to this invention, showinghow the switch-modulator 15 is capable of functioning as a combinedpulse width/pulse position modulator, generating radio frequency signalsfrom base band signals represented by polar coordinates, i.e. theamplitude and the phase. The Cartesian coordinates of a base band signalI+jQ, i.e. the I-signal and the Q-signal, are converted into Polarcoordinates by the converter 10, and the amplitude-signal, A, and thephase-signal, φ, represents the Polar coordinates. The phase signalmodulates a carrier frequency signal into the clock input of themodulator, and the amplitude signal feeds the I-signal input, oralternatively the Q-signal input, of the switch-modulator 15.

In FIG. 12, the illustrated arrangement comprises a switch-modulator 15according to a first embodiment of this invention, having two balancedI-outputs and two balanced Q-outputs. Two unipolar, modulated balancedpulse sequences 3 a, 3 b, are output from switch-modulator 15 andamplified by two power amplifiers 4 a, 4 b. Thereafter, the modulatedand amplified balanced pulse sequences are combined in a differentialcombiner into a bipolar pulse sequence, which is filtered in theband-pass filter 6.

In FIG. 13, the illustrated arrangement comprises a switch-modulator 15according to a second embodiment of this invention, having an unbalancedI-output and an unbalanced Q-output. A unipolar pulse width- and pulseposition-modulated pulse sequence is output from switch-modulator 15 andamplified the power amplifier 4. Thereafter, the modulated and amplifiedpulse sequence is filtered in the band-pass filter 6.

FIG. 14 illustrates an arrangement comprising a switch-modulator 15according to a third embodiment, which performs the combination of theI-signal and Q-signal within the switch-modulator, before theamplification, by generating a combined, unbalanced binary level outputsignal to be forwarded directly to a single switched-mode amplifier 4.In this case, means for restricting the peak levels of the base band I-and Q-signals must be inserted before the input of the switch-modulatorin order to avoid overlapping I- and Q-pulses, e.g. for performingsuitable level back-off or a clipping function (not illustrated in thefigure). In this third embodiment, the switch-modulator can not havebalanced outputs, and it can not handle input signals representing polarcoordinates of the base band signal.

In all of the above-described figures, the filter 6 is shown in aposition after the combination of the I-signal and the Q-signal.However, this position of the filter is only illustrated to give moreclear conceptual view of the invention. In a real implementation, apreferred position of the filter or filters may e.g. be closer to theswitching devices in the power amplifiers, and before the combination ofthe I-signal and the Q-signal.

An advantage with a switch-modulator 15 according to the above-describedembodiments of this invention is that it can have a circuit design thatmay be implemented e.g. as an RF ASIC, and such circuit designs areillustrated in the FIGS. 15 and 17.

FIG. 15 shows a functional block diagram of an exemplary circuit designimplementing a switch-modulator 15 according to a first embodiment,having balanced outputs, and the functional blocks can be implemented indifferent ways, depending on the circuit implementation technology. Thecircuit in FIG. 15 comprises an unbalanced I-input 27 and an unbalancedQ-input 28, and differential balanced signals are created by passing ofthe signals through unity gain inverting and non-inverting amplifiers,as illustrated in the figure, or by any other suitable means. In theQ-signal branch, one of the differential signals Q+ and Q− is selectedalternately and in opposite ways, e.g. by means of two switches. Theswitches are steered from the level of a 50% duty cycle square waveclock signal Q_S, thereby generating the two chopped signals Q1, 32 a,and Q2, 32 b, having opposite phases. The two square wave clocks Q_S andI_S can e.g. be generated by a Schmitt trigger function from thesinusoidal, opposite branch, I-clock signal 25, I_CLK, and Q-clocksignal 26, Q_CLK, respectively, generated by the quadrature clockgenerator 16 incorporated in the circuit. The reference input for thequadrature clock generator 16 is the input clock signal 21, CLK, of thecircuit. The signals Q1, 32 a, and Q2, 32 b and the quadrature-phasesinusoidal Q-clock signal Q_CLK 26 are connected to two comparators 18a, 18 b, where the Q-clock signal is connected to the positive inputs inparallel and the chopped Q-branch signals Q1, 32 a, and Q2, 32 b areconnected to the negative inputs, respectively. The outputs 20 a, 20 b,from the comparators 18 a, 18 b constitute the two differentiallytime-shifted balanced binary-level Q-output drive signals Q_B_P, 30 aand Q_B_N, 30 b, for a switched-mode RF power amplifier.

FIG. 16 illustrates some of the above-described signals generated in theQ-branch, relative 0 Ts, 0.25 Ts, 0.5 Ts, 0.75 Ts and 1 Ts.

The I-branch in the circuit according to FIG. 15 functions in the sameway as the above-described Q-branch, only properly time-shifted by thein-phase sinusoidal I-clock signal 25, I_CLK, and the square wave clockI_S. Accordingly, the outputs 19 a, 19 b, from the comparators 17 a, 17b constitute the two differentially time-shifted balanced binary-levelI-output drive Signals I_B_P and I_B_N 29 a, 29 b, for a switched-modeRF power amplifier.

Note that the clock signals I_S and Q_S used for steering the switchesare in quadrature to the actual signal-branch I-clock signal 25, I_CLK,and Q-clock signal 26, Q_CLK, respectively.

Thus, the I-signal and the Q-signal are processed in separate paths inthe switch-modulator, maintaining the proper phase difference as atime-shift in the output signals by means of the common synchronizingquadrature clock signals. Further, the circuit will generate a correctarcsine pre-distorted differential time-shift around the fixed offsets,according to the above-indicated tables, of the two output pulsesequences, maintaining an approximate 50% duty cycle. This results inlow sensitivity to signal- and clock level-errors. Furthermore, by meansof this circuit design, exact sampling positions corresponding to theresulting pulse positions are achieved, thereby resulting in a very lowdistortion level of the resulting RF signal. Additionally, any resultinglevel imbalance in the pulse width, or phase imbalance in the time-shiftbetween the I- and Q-branches will not lead to any degradation of the IM(Inter-Modulation) performance, only to a degradation of the EVM (ErrorVector Magnitude) performance, due to the final linear superposition ofthe two branches.

FIG. 17 shows a functional block diagram of another exemplary circuitdesign provided with additional circuitry implementing aswitch-modulator 15 according to the second embodiment, having anunbalanced I-output 22 and an unbalanced Q-output 24, as well asaccording to the third embodiment, having an unbalanced combinedI+jQ-output 23. This circuit is implemented by generating unbalancedoutput signals 29 c, 30 c for the I- and Q-branches by a logiccombination of the differentially time-shifted balanced I- and Q signals29 a, 29 b, 30 a, and 30 b, which constituted outputs in the circuitillustrated in FIG. 13 implementing a switch modulator according to afirst embodiment.

In FIG. 17, the XOR-gate receiving the balanced I- and Q-signals 29 a,band 30 a,b, implements a difference function (I_DIFF or Q_DIFF) betweensaid balanced signals 29 a,b (I_B_P and I_B_N) and between the balancedsignals 30 a,b (Q_B_P and Q_B_N), generating a unipolar pulse sequencehaving two pulses in each sample period. The Schmitt trigger function(I_SIGN or Q_SIGN) from the chopped signal (I2 or Q1) together with theAND-gate selects one of these pulses, at the correct position, dependingof the sign of the input I or Q signals.

Additionally, according to the third embodiment, a logic combination ofthe unbalanced I- and Q signals 29 c and 30 c generates a singleunbalanced combined output signal 3 c comprising the combination, i.e.addition, of the I- and Q-branches, forming I+j·Q, which also requires arestriction of the pulse widths in order to avoid overlapping I and Qpulses. The OR-gate 13 combines the unipolar pulse sequences in the I-and Q-branch into one combined pulse sequence 3 c (IQ_C) on the output23.

FIG. 18 illustrates some of the above-described signals generated in theQ-branch, as well as the output signals, relative 0 Ts, 0.25 Ts, 0.5 Ts,0.75 Ts and 1 Ts.

FIG. 19 illustrates a magnified view of a filtered-out spectrum of abase band signal that is switched-modulated and amplified, according tothe method and arrangement of this invention. The filtered-outfrequencies represent the amplified original complex base band signal,and the two-carrier nature of the filtered-out signal indicates that agood dynamic range is achieved.

FIG. 20 is a flow chart illustrating a method of switch-modulating aninput signal to a radio frequency power amplifier, according to anexemplary arrangement of this invention, as illustrated in the FIGS. 7and 14, in which the combination of the I- and Q-branch is performedbefore the amplification.

In the method in FIG. 20, the sampled input base band signal in theCartesian complex coordinates I+j·Q is represented by one I-signalbranch and one Q-signal branch, of which the peak-levels are restricted,in step 210, in order to avoid overlapping pulses. Thereafter, in step220, the I- and Q-signals are input to two separate pulse widthmodulators for mapping of the amplitude of each sample on the width of apulse in a pulse sequence.

A time shift of the pulses representing negative amplitudes relative thepulses representing positive amplitudes is performed in step 230,placing the positive and negative sample pulses on two differentpositions within the sample period, e.g. positive sample pulses on 0.25Ts and negative sample pulses on 0.75 Ts. In order to facilitate asimple combining of the separate I-signal and Q-signal branches again tore-create the complex representation I+j·Q, the pulse sequencerepresenting the I-signal branch is delayed in step 240, e.g. by 0.25Ts, which corresponds to a multiplication with j.

In step 250, the amplified pulse sequences are combined to create acomplex representation I+j·Q, and in step 260, the combined pulsesequence is amplified in a power amplifier. Finally, in step 270, thecombined and amplified pulse sequence is filtered in a suitable bandpass filter, to obtain an amplified base band signal.

This invention is suitable for use in a Voltage Mode Class D (VMCD)switched-mode power amplifier-topology, but it is not limited to be usedin such a topology. Specifically, three variants of VMCD topology mayuse a switch-modulator according to this invention. The first variant isthe double balanced topology, having two H-bridges, one for each of Iand Q and using balanced drive signals, according to the firstembodiment of the invention. The second variant is the balancedtopology, having an H-bridge with one half-bridge for each of I and Qand using unbalanced drive signals, according to the second embodimentof the invention. The third variant is the single-ended topology, alsocalled Class F, using a combined I- and Q-drive signal, according to thethird embodiment of the invention. However, this invention is notlimited to any of these topologies.

The presented switch-modulator according to this invention is suitablefor switched-mode radio frequency power amplifiers, achieving a largepower efficiency and dynamic range and avoiding phase wrap-around andphase jumps. Further, the switch modulator can be implemented with acircuit design suitable for RF ASIC.

Regarding the linearity, the switch-modulator performs sampling of theI- and Q-signals at the correct positions in relation to the actualtime-shifted positions of the output pulses, without requiring anyinterpolation. Further, it performs the required arcsine pre-distortionof amplitude signals with low sensitivity to amplitude errors, andavoids the time granularity problem of the digitally defined pulse widthand position. A switch-modulator according to one embodiment of thisinvention is capable of generating differential (balanced) binary leveloutputs that can be easily combined after the amplification inswitched-mode power amplifiers (PA:s), and thereby the combined signalis insensitive to the linear or non-linear response of the amplifiers,only to differences in the responses between the two amplifiers.Further, the inter-modulation performance is un-sensitive to anyimbalance between the internal I and Q signals and between thequadrature clocks, and to the relative phase shifts of these signals,meaning that low values of spurious or Inter-Modulation (IM) components(<−65 dBc in simulations) will be generated in the output RF spectrum,from zero frequency up to 3 times the carrier frequency for balancedoutput signals, and ideally not further linearization of the PA isneeded.

Regarding the efficiency, the switch modulator according to thisinvention will not inherently restrict the achievable power efficiency.

Regarding the sensitivity, the Error Vector Magnitude(EVM) will beaffected by any imbalance between the internal I- and Q-signals, betweenthe quadrature clocks and the phase shifts of these signals, and by anyimbalance in the gain between the two separate power amplificationbranches for I and Q. However, the sensitivity is low for phase shifts.

Regarding the radio architecture, the inputs to a switch-modulatoraccording to this invention can be connected directly to the output froma Digital-to-Analogue converter with a relatively low sampling rate,after low pass filtering. Further, the embodiment having unbalancedoutputs allows less complicated radio architecture.

While the invention has been described with reference to specificexemplary embodiments, the description is in general only intended toillustrate the inventive concept and should not be taken as limiting thescope of the invention.

The invention claimed is:
 1. A switch-modulator for a switched-moderadio-frequency power amplifier, the switch-modulator arranged to map aphase and amplitude of an input complex baseband signal, represented byan I-signal and a Q-signal, onto modulated output pulse sequences, andsample and modulate the I-signal and the Q-signal separately, amodulation comprising a time-shift of pulse positions within a samplinginterval, the switch-modulator comprising: a first input to a separateI-signal path in the switch-modulator; a second input to a separateQ-signal path in the switch-modulator; and a third input to a quadratureclock generator in the switch-modulator, the quadrature clock generatorbeing arranged to generate an I-clock signal and a Q-clock signal havinga quadrature-shifted phase-relationship in order to delay the modulatedI-signal pulse sequence relative to the modulated Q-signal pulsesequence, and to generate a time-shift of the pulse positions in themodulated output pulse sequences.
 2. The switch-modulator according toclaim 1, wherein the I-clock signal and Q-clock signal are sinusoidal,and arranged to generate an arcsine pre-distortion associated with themodulated output pulse sequences.
 3. The switch-modulator according toclaim 1, wherein the modulation of the I-signal and the Q-signalcomprises mapping of the amplitude of the I-signal and the Q-signal on adifferential time position of two balanced 50%-duty cycle modulatedI-signal pulse sequences and two balanced 50%-duty cycle modulatedQ-signal pulse sequences, respectively.
 4. The switch-modulatoraccording to claim 1, wherein the modulation of the I-signal and theQ-signal comprises mapping of the amplitude on pulse widths for pulsesof the I-signal and the Q-signal, and time-shifting the pulsescorresponding to negative sample values relative to the pulsescorresponding to positive sample values onto an unbalanced modulatedI-signal pulse sequence and an unbalanced modulated Q-signal pulsesequence, respectively.
 5. The switch-modulator according to claim 4,wherein the I-clock signal and the Q-clock signal are arranged togenerate correct time-shifted positions and pulse widths for theunbalanced modulated I-signal pulse sequence and the unbalancedmodulated Q-signal pulse sequence.
 6. The switch-modulator according toclaim 4, wherein a combiner is arranged to generate a combined modulatedpulse sequence, representing a complex I+J*Q-signal, from the unbalancedmodulated I-signal pulse sequence and the unbalanced modulated Q-signalpulse sequence.
 7. The switch-modulator according to claim 4, wherein atime-shift of the pulses corresponding to the negative sample valuesrelative to the positive sample values corresponds to 0.5 of thesampling interval.
 8. The switch-modulator according to claim 1, whereinthe delay of the modulated I-signal pulse sequence relative themodulated Q-signal pulse sequence is 0.25 of the sampling interval,which corresponds to a 90-degree phase-shift and simplifies acombination into a complex I+J*Q-signal.
 9. The switch-modulatoraccording to claim 1, further comprising two I-signal comparators forgenerating two differentially time-shifted balanced modulated I-signalpulse sequences from the I-clock signal and from two opposite-phase andchopped I-signals, and two Q-signal comparators for generating twodifferentially time-shifted balanced modulated Q-signal pulse sequencesfrom the Q-clock signal and from two opposite phase chopped Q-signals.10. The switch-modulator according to claim 9, further comprising twoI-signal outputs for the two differentially time-shifted balancedmodulated I-signal pulse sequences and two Q-signal outputs for the twodifferentially time-shifted balanced modulated Q-signal pulse sequences.11. The switch-modulator according to claim 9, further comprising anI-signal-gate for generating an unbalanced modulated I-signal pulsesequence from the two differentially time-shifted balanced modulatedI-signal pulse sequences, and a Q-signal-gate for generating anunbalanced modulated Q-signal pulse sequence from two differentiallytime-shifted balanced modulated Q-signal pulse sequences.
 12. Theswitch-modulator according to claim 11, comprising an I-signal outputfor the unbalanced modulated I-signal pulse sequence and a Q-signaloutput for the unbalanced modulated Q-signal pulse sequence.
 13. Theswitch-modulator according to claim 1, further comprising aCartesian-to-polar-converter for converting the I-signal and theQ-signal representing an input baseband signal into an A(t)-signalrepresenting an amplitude of the input baseband signal and connected tothe first input, and a phase modulated ω(t)-signal representing a phaseof the input baseband signal and connected to the third input.
 14. Theswitch-modulator according to claim 11, wherein a combiner is arrangedto add the unbalanced modulated I-signal pulse sequence to theunbalanced modulated Q-signal pulse sequence resulting in a combinedmodulated pulse sequence representing a modulated input baseband signalI+J*Q.
 15. A method of switch-modulating a radio-frequency poweramplifier, an input signal represented by an I-signal and a Q-signal ofcomplex components (I+J*Q), the method comprising: sampling and pulsewidth modulating the I-signal and the Q-signal separately to create amodulated I-signal pulse sequence and a modulated Q-signal pulsesequence in a switch-modulator; time shifting pulses corresponding tonegative sample values relative pulses corresponding to positive samplevalues of the I-signal and the Q-signal; delaying each pulse of theI-signal by introducing a delaying time shift; and combining themodulated I-signal pulse sequence and the modulated Q-signal pulsesequence after amplification to create a combined amplified pulsesequence.